Photosynthesis in naturally reared aquatic communities

Introduction

PHOTOSYNTHESIS IN NATURALLY REARED AQUATIC COMMUNITIES

J. VERDUIN

Received April 10, 1950

The productivity of aquatic communities has been the subject of numerous researches and theoretical analyses in recent years. Direct measurements of photosynthetic activity per volumetric unit of phytoplankton, however, are conspicuously scarce in the literature. Students of aquatic photosynthesis have usually expressed their results as "yield per 106 cells" or "yield per liter of natural water." In the present report measurements of photosynthetic rates per unit of phytoplankton volume are presented, and their bearing on aquatic production is discussed.

Plant Physiology 26 (1): 45. (1951).

Methods

The method of measuring photosynthesis described by OSTERHOUT and HAAS (2) was modified by using a Beckman pH meter instead of colored indicators for pH determination. The equilibrium equations published by MOORE (1) in his graphic method of determining free carbon dioxide, bicarbonate, and carbonate concentrations in natural waters form a basis for determining carbon dioxide removal during photosynthesis (or evolution during respiration) by making pH measurements at the beginning and end of an experiment. The graph in figure 1 shows the relation between carbon dioxide removal and pH-change in water of a given total alkalinity (90 p.p.m.) between pH values of 7.7 to 9.5. When a Beckman pH meter is used the associated C02-change can be measured with an error of less than 10%, if the pH-change amounts to 0.30 pH units or more.

The phytoplankton was collected with a 10-liter Juday trap, the samples were transferred to bottles having a volume of about 30 ml. and placed in a dark compartment until the collecting trip was completed (two to four hours). On arriving at the laboratory the samples were diluted with lake water which had been filtered through silk bolting cloth of the same type used on the trap (0.03-0.04 mm.-mesh). The degree of concentration to which the natural populations were subjected varied with their density in the lake. The highest populations observed, 2.5 x 106 standard volumetric units per liter (8x 103 cubic micra equals 1 unit, see WELCH (4)), were made up to concentrations of five times their natural density. The lowest populations observed were made up to 400-fold concentrations. The experimental concentrations, however, were so low that light transmission through a test flask was never more than a few per cent. lower than for a control flask containing filtered lake water. No attempt was made to exclude zooplankton.

Subsamples of the phytoplankton concentrate were enclosed in 125-ml. Erlenmeyers for the photosynthesis test. The flasks were placed in a tray through which lake water was circulated to maintain them near lake temperature, and were illuminated with fluorescent light of about 400 footcandle intensity. Tests showed this intensity to be close to the optimum for photosynthesis of the Tabellaria community which developed in the spring of 1949. The pH was determined at the beginning of an experiment, and again after two hours of illumination. If the pH-change was less than 0.3 units the experiment was continued, but was usually ended within four hours because sharp reductions in rate occurred when tests were prolonged beyond four hours; the rates approaching zero after eight hours of illumination. Such samples, however, recovered their initial rate after being stored in the dark overnight.

Phytoplankton counts were made, on fresh (unformalinized) samples, soon after the photosynthesis tests were completed. Ten fields were counted for each sample; see WELCH (4) for details of counting procedure. Threedimensional measurements were made of the major components of the populations studied providing volumetric units for expressing photosynthetic rates.

Results

Measurements of apparent photosynthesis (excess over respiration) were made on -plankton samples taken from westem Lake Erie at approximately biweekly intervals during 1949. Figure 2 shows the apparent photosynthesis of the phytoplankton from one liter of water graphed against the volumetric units per liter present in the lake. In most cases each datum point represents the average of six determinations, samples collected on the same day from three depths (0-, 5-, and 9-meters) at two stations three miles apart. The solid line represents the regression line, indicating a rate of 1.5 micromoles of CO2 absorbed per hour per 106 units of phytoplankton. The genera which contributed importantly to these data were Tabellaria, Melosira, Pediastrum, and Fragilaria. The test temperature, being near that of the lake, varied between 1-27° C during the year. The correlation is high (coefficient of .96) but the scatter between 0 and 1 on the abscissa is obscured by the excellent fit on the high values. A separate correlation computed for the scatter diagram between 0 and 1 yielded a coefficient of .92, and a regression line (broken) indicating a rate of 1.1 micromoles of CO2 absorbed per 106 units per hour. The broken line includes significant contributions from zooplankton respiration, which appears as reduced photosynthesis, but zooplankters were conspicuously scarce in the dense Tabel-laria community. It is noteworthy that the highest zooplankton populations present did not exhibit respiration rates which exceeded photosynthesis, but apparent photosynthesis consistently showed positive values. Respiration rates were determined on about two-thirds of the samples studied, making an estimate of the average total photosynthetic rate possible. The apparent rate in 87 tests amounted to 86% of total photosynthesis, so respiration, including that of the zooplankton, averaged only 14% of total photosynthesis. The respiration rate of the dense Tabellaria community, however, which contained negligible amounts of zooplankton, amounted to 12%fo of total photosynthesis. So the apparent rate of 1.5 micromoles CO2 absorbed per 106 units of phytoplankton per hour must represent a total rate of about 1.7.

Discussion

The method of measuring photosynthesis described above has three advantages which recommend it to students of aquatic photosynthesis: (1) it does not require pretreatment of the water, or elaborate precautions to avoid oxygen contamination, as methods of measuring oxygen evolution do, (2) the measuring technique leaves the samples unchanged so repeated observations can be made on the same sample, and (3) it is a simple, rapid method which requires no elaborate equipment. Although artificial light was used in the present study the method can readily be adapted to experiments under natural light.

The data presented above represent an attempt to measure the photosynthetic activity of the communities arising naturally in western Lake Erie. Other methods have been devised to obtain such information, notably the clear and blackened bottle technique used by RILEY (3). (He has also applied the technique widely in studies of marine production.) The present report affords a marked contrast, in two aspects, to his 02-production values obtained in Linsley Pond: (1) the high correlation between C02-absorption and phytoplankton volume reported here as compared to a statistically insignificant correlation between standing crop and 02-production in Linsley Pond, and (2) the high rate of apparent photosynthesis (86%o of total photosynthesis) reported here, as compared to a negative value for apparent photosynthesis (02-consumption exceeded 02-production) in Linsley Pond. It is obvious that Riley's experiments differ in several respects from those reported here. Both methods, however, are attempts to isolate the primary producers under approximately natural conditions. Whether concentrating the populations and subjecting them to artificial light for a few hours is a greater departure from the natural than confining unconcentrated populations in clear and blackened bottles for seven days is a debatable question. It seems logical, however, to expect that a rather good correlation exists between natural photosynthesis and phytoplankton volume, and that apparent photosynthesis is usually positive in natural waters. If the latter were not true there would be no organic matter deposited on the bottom. The high correlation evident in figure 2 suggests that the major components of the communities which are adapted to a particular aquatic habitat exhibit closely similar photosynthetic rates per unit. of plant volume. This observation has an important bearing on aquatic production. Many students of productivity consider the standing crop of phytoplankton an unreliable index of production. This concept contains the tacit assumption that photosynthetic rates per unit volume vary widely with varying conditions and different organisms. The present study suggests that such variations may not be so erratic as has been assumed, and that the standing crop, since it represents the photosynthetic equipment on which production is based, may be worthy of reconsideration as an index of productivity. The average annual standing crop, at least, when based on a sufficient sampling frequency and expressed in volumetric phytoplankton units, may provide a fairly satisfactory criterion for estimating the base of the production pyramid.

Summary

1. A method of measuring photosynthesis, using pH-change as a measure of C02-removal from natural water, is recommended for studying naturally reared plankton communities.

2. A study of the communities arising in western Lake Erie during 1949 (using fluorescent light of 400 fc) showed a high correlation between volume of phytoplankton and photosynthetic rate, indicating that the major components of these communities were much alike in photosynthetic activity. The total photosynthesis per volumetric unit of phytoplankton was about 1.7 micromoles of C02 absorbed per 106 standard units per hour (unit is 8 x 103 cubic micra).

3. The data suggest that the standing crop of phytoplankton, when expressed in volumetric units, may be a fairly reliable index of productivity.

The author wishes to acknowledge that the work was facilitated by the encouragement and aid of Dr. T. H. Langlois, director of the. Institute, and Edward Kinney, his assistant.

Literature Cited

1. MOORE, E. W. Graphic determination of carbon dioxide and the three forms of alkalinity. Amer. Water Works Assoc. Jour. 31: 51-66. 1939.